Rheostat



June 25, 1946. E. c. RAGATZ- RHEOSTAT 2 sheets-sheet 1 Filed Marqh 20,1945 INVEN TOR.- EUGENE C. RAGATZ 1943 v2 Sheets-Sheet 2 June 25, 1946.E. c. RAGATZ RHEOSTAT Filed March 2O INVENTOR. EUGENE C RAGATZ NN mw RNIlllllll-IIHIIII ATTORNEY Patented June 25, 1946 UNITED STATES PATENTOFFICE RHEOSTAT Application March 20, 1943, Serial No. 479,949

4 Claims. 1

This invention relates, in general, to rheostats and specifically torheostats wherein the resistancey change is obtained by variation ofpressure upon a pile of individual resistors.

This invention resides in an improved form thereof in which the ohmicresistance, in response to a given applied pressure to the pile ofindividual resistors, is predictable and consistant, the same resultingfrom control of the atmosphere surrounding the individual resistors.

It is common practiceto use individual resistors such as plates ofcarbon arranged in a pile or column. The plates are contained in aninsu* lated rack or in an insulated tube with the faces of the plates incontact with each other and a screw or plunger device is arranged toapply the pressure at4 one end of the column. A particular type of sucha pressure rheostat employs plates made of graphite, coated withamorphous carbon, which plates provide a low internal resistance and ahigh surface or contact resistance. With little pressure upon the pileof the plates the resistance is nearly all made up of surface contactresistance and with high pressure upon the pile the resistance ispractically only that of the internal resistance of the plates. Anothertype of pressure rheostat utilizes plates homogeneous throughout ofamorphous carbon.

Amorphous carbon when used in the prevailing atmosphere may undergo achange in its contact resistance dependent upon the extent of access ofthe atmosphere to the surface of the carhon and the degree f temperatureof the car bon. Such changes may not be oi marked degree and may wellb'e permissible in the rheostat for the ordinary control of electricalmachines and devices. There is, however, a demand for rheostatic controlwherein a precise control is required and wherein the resistance must bein close limits when a pre-determined pressure is applied. It is in thelatter field that my invention applies.

It is a' commercial practice to form the carbon as used in theindividual plates of the pressure rheostat at temperatures in theneighborhood of 1500 F. The carbon is formed in pots or crucibles placedin a gas-fired furnace, which uses gas made from coal. The carbon in itsformation is kept in the furnace for a period long enough to have thecarbon in a iixed state so far as temperature is concerned.

I have discovered that carbon formed in this manner and when used in arheostat from which oxygen is excluded will not change in contactresistance, if the temperature is kept below the formation temperatureof the carbon. Likewise such carbon placed in a vacuum and operatedbelow the formation temperature will not change to an' appreciabledegree in its Contact resistance. It is understood, of course, thatcarbon has a negative temperature coefficient, and change in resistancedue to temperature alone is not considered. My invention thereforeprovides for a rheostatin which the measure of resistance is dependentupon pressure applied and in which like pres sures will cause likeresistance to be repeated.

It is an object of this invention to remedy the shortcoming and fault ofa particular type of change in resistance of amorphous carbon as used inthe pressure type rheostat.

It is a further object of this invention to provide a rheostat device ofthe pressure type of such capacity and of suchconstruction that it maybe used for the percision c ontrol of motors and other electricaldevices in the usual industrial applications.

It is a still further object of this invention to provide a rheostatdevice of the pressure type utilizing carbon type discs, which discs areprotected from the access of oxygen.

An example of my invention is hereinafter more particularly described byreference to the accompanying drawings forming a part hereof.

The several gures of the drawings are briefly described as follows:

Figure l is an assembly View in side elevation of a rheostat constructedin accordance with this invention; v

Figure 2 is a perspective view of an individual resistor in the form ofa graphite disc coated with amorphous carbon;

Figure 3 is a transverse sectional view of a fragment of the disc shownin Figure 2;

Figure 4 is a transverse sectional View of a fragment of a disc composedentirely of amorphous carbon;

Figure 5 is a longitudinal sectional view of a gas-tight joint as usedin the construction of the rheostat;

Figure 6 is a longitudinal sectional view of the end of the rheostat onwhich pressure is applied and as shown in Figure 1; and

Figure '7 is a longitudinal sectional view of the stationary end of therheostat as shown in Figure 1.

As depicted in the drawings the rheostat proper housed in the steel tubel is mounted in a U-n shaped frame 2, forming an assembly arranged toreceive pressure in the direction indicated by the arrow in Figure 1.The steel tube l is provided with a ceramic insulating lining I5 whichreceives individual resistors 40 of the type shown in Figure 3 or Figure4 to be described in more detail later.

Referring now more particularly to Figure 6, a pressure plunger I8 isshown mounted within a corrugated flexible metal tube I1 with its innerend in electrical connection with the end of the pile of resistors 40and with its outer end in electrical contact with a flexible jumperconnection I0. A metal plug I9 is attached by a gas-tight connection tothe inner end of the flexible tube Il. The outer wall of the metal plugI9 is attached with a gas-tight connection to a metal thimble 6. Theouter end of the flexible metal tube I1 is attached to a metal plug 20which, in turn, is attached to the pressure plunger I8; both of saidlatter two attachments being gas-tight. The steel tube I is providedwith a hermetically joined extension in the form of a metal sleeve I Iwhich provides a support for a gas impermeable porcelain tube 4, thelatter being connected to metal thimble 6 as shown. The jointconstruction between metal sleeve II and porcelain tube 4 and the jointconstruction between metal thimble 6 and porcelain tube 4 is impermeableto gas and will be described in detail later on. The pressure plunger I8is drilled and threaded to receive the stud 24 formed as a part of themetal socket 26. The flexible jumper I is fastened to metal socket 26 bymeans of nut 25 and thus brought into electrical connection with theplunger IB. Within the metal socket 26 there is a porcelain button 21 towhich pressure may be applied as indicated in Figure 1.

The construction of the stationary end of the rheostat is shown indetail in Figure 7 in which a metal sleeve I2 forms an extension of thesteel tube I, thus providing a support for a gas impermeable porcelaintube 3. To the outer end of porcelain tube 3 is attached by gas-tightconnection a metal thimble 5. On the end of the pile of discs 40 is ametal contact disc I i5 to which is attached flexible conductor I4, theother end of which conductor is attached to a stud I3. The stud I3 isattached to metal thimble 5 by a gastight connection which, in thiscase, is a silver solder joint not shown. The metal thimble is threadedon its end portion, as shown, to allord connection to a circuit.

The metal thimble 5, the metal sleeve II, the metal sleeve I2, and themetal thimble are attached to their respective porcelain tubes 4 and 3by a construction as shown in detail in Figure 5. Illustrating this, theporcelain tube 3 is coated with a copper film 23 which, in thisparticular construction, is sprayed on. The metal thimble 5 or thesleeve, as the case may be, is then placed over the end of the porcelaintube and the intermediate space is then filled with solder 22 by asoldering operation. This type of juncture has been found to begas-tight.

Referring to Figure 1, the electrical connection is completed from thejumper I0 to bolt 8, which is secured into the U-shaped frame 2 by thebushing of insulating material 9.

With pressure applied to the porcelain button 21 as indicated in Figure1, the pressure plunger I8 moves inwardly displacing the resistor discs40 a slight distance depending upon the length of the column of discsand the thickness of the discs. The pressure plunger I8 is preventedfrom being moved outwardly further than necessary by shoulder 2| formedon plunger I8 as shown,

Referring to Figures 2, 3, and 4 the carbon type disc is shown in Figure2 and a fragment in trans- Verse section in Figure 3. This type of dischas a graphite interior 42 and a surface carbon coating indicated as at4I. In Figure 4 a fragment of the disc is indicated at 43, which is acarbon type disc with carbon material throughout. The type of disc usedin the rheostat as described depends upon the electrical service inwhich the rheostat is used and also upon the over-all resistance rangerequired of the rheostat.

It appears that the contact resistance of the pressure type rheostatdepends largely on the Volatile content of the carbon surfaces.

By contact resistance is meant the resistance developed to the passageof current between the contacting or substantially contacting surfacesof the carbon.

This dependence of Contact resistance on volatile content is explainedby the fact that the volatile matter is adsorbed on the carbon surfacesand pores in the form of rather tightly adherent lilms under certainconditions. These films control the closeness with which one piece ofcarbon can approach another piece with the applied pressure heldconstant. The thinner the film, the closer the approach and the lowerthe resistance and vice versa.

The volatile matter of such carbons might be composed of many substancesdepending on the complete history of the carbonthat is, a more or lessactive carbon used for selective adsorption of say lethal gases wouldhave a volatile matter largely composed of these gases and in anapplication for adsorption of heavy hydrocarbon gases from natural gasit follows that the volatile matter is largely hydrocarbons. However, inthe manufacture and use of carbons as applied to resisters, the volatilematter is composed largely of a complex mixture of compounds of carbonand oxygen, water, and other gases whose presence and amounts depend onthe manufacturing conditions,

The success of my invention is probably attributable to the formation ofcarbon elements having a comparatively Small amount of adherent film ofvolatile matter of a fixed character, and the enclosure of such carbonelements Within a gas tight chamber containing an inert gas.

While this invention has been above described by reference to a specificinstance of an embodiment thereof, it is intended that the protection ofLetters Patent to be granted hereon be not unnecessarily limitedthereby, and that such protection extend to the substance of the advancecontained in this disclosure anddeflned in the claims hereto appended.

What I claim as my invention is:

1. In a compressible pile rheostat, a hermetically sealed casing havinga deflectable wall, an inert atmosphere within said casing, a pluralityof contiguous conducting elements having surfaces Of amorphous carbon,said elements forming a pile, an insulating surface forming a supportfor said pile within said casing, electrodes having connectionsexternally of said casing engaging the ends of said pile, and pressureproducing means adapted to cause said electrodes to be forced toward oneanother with accompanying deflection of said defiectable wall and toapply pressure to said pile.

2. In a compressible pile rheostat adapted to preserve consistentpressure-resistance relationship at constant temperature the combinationcomprising a pile of heat stabilized carbon containing elements, anon-deiiectable housing portion enclosing said pile and composed of asubstance permanently impervious to gaseous diffusion, an electricallyconducting stationary terminal in engagement with one end of said pile,a deflectable housing portion composed of material permanentlyimpervious to gaseous diffusion joined to said non-deflectable housingportion by a joint permanently impervious to gaseous diffusion, anelectrically conducting movable terminal in engagement with the oppositeend of said pile insulated from said rst named terminal and adapted tobe forced towards said first named terminal to apply pressure to saidpile with accompanying deflection of said deflectable housing portion,and an atmosphere of gas nonreactive with respect to carbon permanentlymaintained within said housings` and surrounding said pile.

3. In a compressible pile rheostat adapted to preserve consistentpressure-resistance relationship at constant temperature the combinationcomprising a pile of heat stabilized carbon containing elements, anon-deflectable housing portion enclosing said pile composed of asubstance permanently impervious to gaseous diffusion, means within saidhousing adapted to provide an insulated guide support for maintainingsaid carbon elements electrically distinct and in pile formation, anelectrically conducting stationary terminal in engagement with one endof said pile and extending outwardly through said non-deflectablehousing portion to the exterior thereof through a joint permanentlyimprevous to gaseous diffusion, a deflectable housing portion composedof material permanently impervious to gaseous diffusion joined to saidnon-deflectable housing portion by a joint permanently impervious togaseous diffusion, an electrically conducting movable terminal inengagement with the opposite end of said pile insulated from said firstnamed terminal and extending outwardly through said deflectable housingportion to the exterior thereof through a joint permanently imperviousto gaseous dif- CTI fusion, said movable terminal being adapted to beforced towards said rst named terminal to apply pressure to said pilewith accompanying deflection of said deectable housing portion, and anatmosphere of gas non-reactive with respect to carbon permanentlymaintained within said housings and surrounding said pile.

. 4. In a compressible pile rheostat adapted to preserve a consistentpressure-resistance relationship at constant temperature the combinationcomprising a pile of heat stabilized carbon containing elements, anon-deectable housing portion in the form of a metallic turbepermanently impervious to gaseous diffusion enclosing said pile, aninsulating lining in said tube adapted to provide an insulating guidingsupport for maintaining said carbon containing elements electricallydistinct and in pile formation, an insulating bushing composed ofmaterial permanently impervious to gaseous diffusion in one end of saidtube joined thereto by a fused joint permanently impervious to gaseousdiffusion, an electrically conducting stationary terminal in engagementwith the end of said pile adjacent said bushing and extending outwardlytherethrough and joined thereto by a fused joint permanently imperviousto gaseous diiusion, a second insulating bushing formed of materialpermanently impervious to gaseous diffusion in the opposite end of saidtube and joined thereto by a fused joint permanently impervious togaseous diffusion, a deectable metallic bellows permaently impervious togaseous diffusion secured to said second named bushing by a fused jointpermanently impervious to gaseous diffusion, a movable electricallyconducting terminal in engagement with the other end of said pile andextending outwardly therefrom through said bellows through a fused jointpermanently impervious to gaseous diffusion, and an atmosphere of gasnon-reactive with respect to carbon permanently maintained within saidhousing bushings and bellows surrounding said pile.

EUGENE C. RAGATZ.

